This invention relates generally to autopilots and more particularly relates to a marine autopilot having an adaptive gain feature.
As is known in the art, a marine autopilot is used to maintain a ship, or vessel on a fixed course while the vessel encounters environmental variations such as changes in wind speed and direction, and changes in sea conditions. Preferably, the vessel course is maintained with minimum intervention by the operator of the vessel. In particular, the autopilot adjusts the position of the vessel's rudder in order to compensate for course deviations caused by changes in, inter alia, waves, wind, currents, and vessel speed.
Some marine autopilots use a proportional plus integral plus derivative (PID) control law to maintain the vessel on a desired course (i.e. during course keeping operation) and a proportional plus derivative (PD) control law to change the course of the vessel (i.e. during course change operation). Such an autopilot provides an output signal, hereinafter referred to as a rudder control signal, which corresponds to a desired change in the position of the rudder. During course keeping operation, the rudder control signal is proportional to the summation of the following terms: an error signal (i.e. the difference between a desired course and the actual instantaneous vessel heading), the time integral of the error signal, and the time rate of change of the error signal. Whereas, during course change operation, the rudder control signal is proportional to the summation of the error signal and the time rate of change of the error signal.
More particularly, each term of the conventional PID and PD control laws has a gain value associated therewith. The gain value associated with the error signal may be referred to generally as a proportional gain value, the gain value associated with the time integral of the error signal may be referred to generally as a rudder bias value, and that associated with the time rate of change, or derivative of the error signal may be referred to generally as a counter rudder value. Thus, during course keeping operation for example, the rudder control signal (i.e. the course keeping rudder order signal) is equivalent to K.sub.p e(t)+K.sub.d e(t)+K.sub.i .intg.e(t)dt, where e(t) is the error signal, K.sub.p is the proportional gain value, K.sub.i is the trim value, and K.sub.d is the counter rudder value.
In such a control system, the proportional term (i.e. K.sub.p e(t)) provides rudder movement proportional to the error signal. The derivative term (i.e. K.sub.d e(t)) provides damping in the sense that once the vessel yaws, the derivative term provides resistance to such motion, or angular velocity. In this way, the derivative term reduces overshoot of the vessel past the desired course. The integral term (i.e. K.sub.i .intg.e(t)dt) provides compensation for low frequency disturbances, such as wind, by providing a bias on the rudder position to offset the effect of such disturbances. Generally, the rudder control signal provided during course change operation (i.e. course change rudder order signal) is as described above with the exception that the integral term (i.e. K.sub.i .intg.e(t)dt) is nulled, or excluded, thus resulting in proportional plus derivative control.
As is also known in the art, autopilots often include manual adjustment capabilities for modifying the various gain values described above, as well as other control system parameters. For example, autopilots may include a manually adjustable counter rudder gain value feature with which the operator of the vessel can adjust the counter rudder gain value in order to correspondingly adjust the resistance of the vessel to angular velocity. The capability of adjusting the counter rudder gain value is desirable since, when the vessel is heading into the waves, some amount of yawing is unavoidable. In other words, performance of the autopilot will no be notably improved by resisting the angular velocity of the vessel in such conditions. However, wear on the rudder and associated drive apparatus can be reduced by reducing the counter rudder gain value in such conditions.
However, manual adjustments somewhat defeat a primary purpose of an autopilot; namely, to maintain a fixed vessel course with minimal operator intervention. Further, such adjustments are not trivial and may be difficult for one unskilled or inexperienced in boating and/or autopilot operation. Thus, although the basis for providing a manual counter rudder gain value adjustment feature is to improve the efficiency of autopilot performance or more particularly, to reduce wear on the rudder and associated drive apparatus by reducing ineffectual rudder motion, such rudder efficiency may actually be degraded by incorrect or excessive adjustments.